This invention relates to the field of visually inspecting ferrous and non-ferrous workpiece surfaces.
The use of visual inspection techniques for determining the condition of a smooth work surface is a common practice in the metal art. Such visual inspection can be used to ascertain the condition and quality of the surface being inspected and can also be used to detect the presence of deep-seated flaws as might result from fatigue cracking, manufacturing defects or other potential failure mechanisms in a workpiece. The detection of failure initiation sites is, of course, of vital importance in the maintenance of aircraft and in the vertification of aircraft components such as wing spar members, engine mounts, airframe surface components, and other highly stressed load carrying members. In industrial and aerospace applications, the use of visual inspection aided by a magnetic field and a suspension of magnetic particles has been routinely employed with ferrous members such as engine components and machine elements; such magnetic techniques are, however, ineffective for the non-ferrous metals commonly used in many structural elements.
The patent art includes several examples of visual inspection techniques wherein media of diverse types are employed as an inspection aid; this art includes the patent of Orlando G. Molina, U.S. Pat. No. 3,826,917, which concerns a magnetic inspection arrangement, the use of non-fluorescent magnetic particles suspended in a liquid medium, and visual inspection under fluorescent or ultraviolet light to provide an image contrasted against a fluorescent background. The use of magnetic particles contrasted against a fluorescent background achieves a high sensitivity inspection arrangement at lower cost than does the background absent fluorescent magnetic particle method that preceded the Molina patent. The Molina apparatus also includes a disclosure of suitable materials for use as a paint or colored coating covering for a material to be inspected. Since a modified version of the Molina color coating can also be employed with the present invention, the disclosure of the Molina patent is hereby incorporated by reference into the present specification.
The patent art also includes the method of detecting surface discontinuities described in the patent of Taber de Forest et al, U.S. Pat. No. 2,499,467, which is concerned with the detection of surface discontinuities in workpieces having relatively low electrical conductivity. The de Forest invention contemplates the use of electrostatic forces and the action of such forces on charged particles of small dimension located in an oil, alcohol or other liquid suspension. The de Forest invention also contemplates use of air suspended particles and the application of suspended particles by means of a brush. In the de Forest invention, a conductive backing member located on the back side of the object being tested is imparted with an electrostatic charge. This charge provides an attraction for charged, liquid suspended particles and the magnitude of this attraction varies at the locus of surface discontinuities in the workpiece.
Another patent, issued to Taber de Forest, U.S. Pat. No. 2,678,420, discloses the use of finely-divided particles and background enhancement for detecting flaws in a workpiece. This patent uses electrostatic attraction of the particles and physical disturbance of the workpiece, as by striking with a hammer for example, in order that mechanical movement of flaw edges can displace an otherwise uniform layer of particles and identify the flaw location.
The inspection patent art also includes the patent of Adolf Mlot-Fijalkowski et al, U.S. Pat. No. 4,321,534, which concerns a magnetizable workpiece, fluorescent magnetic particles, contrasting colors, a liquid suspension and a hydrophobic coating having no affinity for water, used in the detection of tears and faults in a workpiece. Both the workpiece and the applied particles are magnetic in nature in the Fijalkowski invention.
The patent art also includes the invention of Kenneth P. Borrows, in U.S. Pat. No. 4,341,997, which describes a particle inspection arrangement using a coated ferromagnetic particle suspension illuminated by simultaneously applied ultraviolet and white light. A notable feature of the Borrows invention is an ability to provide relatively bright indications of flaw presence and thereby enable inspection without the greatly diminished ambient lighting commonly required in fluorescent inspections.
The use of an electrical current flow in a fatigue crack detecting arrangement is shown in the patent of Nelson A. Crites et al, U.S. Pat. No. 3,803,485, wherein fatigue cracks are automatically located by crack propagation through a detector coating material. Crites also uses a conductive solution that propagates between a fatigue cracked metal member and a conductive layer comprising part of a detection coating. According to the Crites et al invention a reservoir of conductive liquid is fabricated into the detection coating and the rupture of this reservoir by a propagating fatigue crack provides a conductive path across an insulating barrier in the detecting coating.
The inspection art also includes U.S. Pat. No. 3,097,337, issued to H.S. Polin, concerning electroluminescent non-destructive flaw detection. The Polin invention contemplates use of a particulate material such as a metallic sulfide applied to a workpiece either by dusting or in a volatile hydrocarbon vehicle such as alcohol and energized into the light emitting state by a changing electrical field. The Polin invention also contemplates collection of electroluminescent particles within a surface flaw and their production of light energy in response to changes in the applied electrical field. The Polin electrical field changes may result from pulsating DC energy or alternating current energy; the location of these electric fields is controlled by movement of portable current applying probes around the surface of the workpiece being inspected.
The use of a liquid crystal device in combination with the electric field perturbations resulting from surface flaw conductivity variations in a workpiece is not disclosed in these examples of prior inspection techniques.